This is unscientific, and is probably downright stupid, so I am already regretting suggesting it. However, in 5th gear, the engine is turning over faster, flywheels turning faster, etc. The increased inertia may help the bike remain more stable, although I have never noticed this with my bike. Then there is the issue of engine counterbalancers, if your bike has them also. Could something having to do with out of spec flywheels and / or counterbalncers contribute to this feel? As you say, your speed and other variables remain constant. Has your bike always done this? Incidentally, at 70 mph, I always feel wobbly, but its not the bike...